Tandem x-ray tube stabilizer



Dec. 27, 1949 w. N. LUNDAHL ET AL 2,492,304

TANDEM XRAY TUBE STABILIZER Filed May 8, 1948 I n 0 a a o 0 zzz WITNESSES:

INVENTORS Haber-z L. Wrighz and W Walzeray Lundahl.

ATTORNE Patented Dec. 27, 1 949 UNITED STATES PATENT OFFICE inghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 8, 1948, Serial No. 25,954

4 Claims.

ur invention relates to an arrangement for insuringmaintenance of a Constant ratio between the anode currents of two X-ray tubes or other hot cathode electrical discharge tubes fed with alternating current. For certain purposes, such as operating the two X ray tubes of the X ray thickness gauge, such as is describedin Walter N. Lundahl application Serial No. 25,953 for an X-ray thickness. gauge, filed concurrently herewith, it is desirable to maintain a constant ratio between the anode currents of two hot cathode electrical discharge devices under circumstances in which the ratio of these anode currents would normally tend to vary fromtiine to time by reason of changes in the electron emissivity or the cathodes or in thevoltages used to energize the cathodes or the anodes. Our invention provides a circuit network by which the cathode temperatures are variedto compensate for any deviation from a predetermined value in the above-mentioned ratio. In particular, for the Lundahl X-ray thickness gauge, the ratio is to be maintained at a constant value of unity.

One object of our inventionis, accordingly, to provide a novel and improved arrangement for maintaining a constant ratio between the anode currents of two hot cathode electrical discharge tubes.

Another object of our invention is to provide an improved arrangement for Varying the heating current in thecathode of one of a pair of hot cathode electrical discharge tubes to maintain a constant ratio between their anode currents.

Still another object of our invention is to provide an improved arrangement for simultaneously operating a pair of X-ray tubes to maintain a constant ratio between the magnitudes of their anode currents.

Other objects of our invention will become apparent upon reading the following description, taken in connection with the drawing, in which the single figure shows a pair of X-ray tubes operated from a source of alternating current voltage and provided with a regulating device embodying the principles of our invention.

\ Referring. in detail to the drawing, a pair of X-ray t'ub'es A and B is arranged to be energized irom a single alternating-current source 8. The X-ray tubes Aand B are preferably similar to each other and are provided with electron-emissive cathodes heated, respectively, by alternating currents from a pair of secondary windings 5 and IS. The anodes of the X-ray tubes A and B are, respectively, supplied with voltage by a pair of secondary transformer windings Gand I6, each former primaries I and I1.

having one end grounded. Energy is supplied from the alternating-current source 8 to the secondary winding 5 through aprimary winding I I1 and to the secondary winding I5 through a primary winding I I, the latter of which is connected to receive energy from a variable tap on the transformer II8 energized by the alternating current source 8. The secondary windings 6 and I5 are, respectively, energized from the alternating cu'r-rent source 8 through the agency of primary windings I and I1 energized from said source througha primary winding I 2 I Thus the cathodes 0f the X-ray tubes A and 'B are, respectively, connected to the grounded terminals of the transformer secondaries B and I6 which supply these tubes with anode voltage through control windings IIH and I02 positioned on the magnetic cores of the respective trans- In series with the windings WI and I02 are two equal resistors I03 and I04 having their opposite terminals connected to the anodes of a pair of substantially identical diode rectifie'rs which may conveniently be embodied in an electron tube I05. The cathodes assoc-iated in the tube I05, respectively, with the above-mentioned anodes are connected to ground through a pair of similar Variable resistors I01 and I08 which are, respectively, shunted by'capacitors I09 and H0.

One cathode of the double diode I05 is connected to the cathode of an amplifier tube III having an anode supplied with current from a voltage source II2 through a winding H3 positioned on a magnetic core I I3A to control the degree of saturation thereof. The control electrode ofthe tube III is connected to the other cathode of the double diode I05 through a resistor I I4 and a bias battery I I5.

It will be evident, upon examination, from the aboveldescription that the anode current of the x ra'y tube A will flow to earth through the resistor I03 and the channel which is in parallel with resistor I03 and which comprises the righthand diode in the tube I05 and the resistor I01. Similarly, the anode current of the X-ray tube B will flow to earth through the resistor I04 and the channel comprising the left-hand diode in the tube I05 and the resistor I08. It will be clear that when the anode current of the X-ray tubes A and B are equal to each other, the potential d-ropsin the resistors I 03 and I04 are equal and opposite to each other and correspondingly the potential difference betweenthe opposite terminals of the resistors I01 and I08 is zero. The latter potential difference, in series with the bias voltage source H and the resistor H4 is impressed between the cathode and the control electrode of the amplifier tube III and will fix the value of the direct current in the winding H3 which controls the saturation of the magnetic core I I 3A for the winding I IS. The said core and the winding H6, accordingly, constitute a saturable reactor controlled in value b the direct current through the tube I I I and variation of the latter will, accordingly, control the voltage impressed on the primary winding II! from the source H8. The net result of the foregoing situation is that the potential diiference between the opposite terminals of the resistors I01 and I08 will control the cathode temperature of the X-ray tube A.

The initial value of the heating current for the cathode of either the tube A or B may be initially adjusted by varying the voltage sources H2 or H5 or by varying the tap point on the transformer I I8 so that the anode currents of the X-ray tubes A and B have the desired equal values at which they are to be maintained. If thereafter the anode current of the tube A increases above the value of the anode current of the tube B a potential difference will be produced across the outside terminals of the resistors I01 and I08 which will make the control grid of the tube III less positive thereby decreasing current flow through the winding H3. The winding is so poled as to decrease the saturation in the magnetic core for the winding H6 thereby increasing its reactance and decreasing the heating current for the cathode of X-ray tube A. In consequence, the anode current of the tube A will decrease to substantial equality with that of the X-ray tube B.

Should the anode current of the X-ray tube A become smaller than that of the X-ray tube B, the opposite effects on the amplifier tube I I I and the reactance of the winding H6 to those above described will obviously follow, with the result that the anode current of the X-ray tube A will be raised to equality with that of the X-ray tube B. Our stabilizer is thus an effective agency to maintain equality of currents at all times through two electron tube circuits, such as those of the X-ray tubes A and B. It will be noted that the interposition of the diode rectifiers in tube I05 ahead of the resistors I01 and I08 insures that any alternating current due to distributed capacitance between the windings of the transformers supplying power to X-ray tubes A and B will be excluded from effect on the grid potential of the Y amplifier tube III. filter out any ripples in the voltages impressed by the anode currents of the two X-ray tubes A and B across the resistors I01 and I08.

While we have described the anode currents of the X-ray tubes A and B as maintained at a constant ratio of unity, it will be obvious that any other ratio may be maintained between these anode currents by appropriately changing the value of the resistors I03 and I04. Thus, if it is desired that the ratio of the anode current in the tube A to the anode current in the tube B shall be maintained at a constant value N, the ratio of the value at the resistor I 04 to resistor I03 should be N.

It will likewise be obvious that while we have described our invention as applied to control of the anode currents of X-ray tubes, it may be applied to the control of the anode currents of any electrical discharge tubes having electricallyheated thermionically-emissive cathodes.

The capacitors I09 and H0 We claim as our invention:

1. In combination with a pair of X-ray tubes each having a thermionically-emissive cathode heated by electric current, a circuit supplying heating current to one of said cathodes and embodying a reactor having a saturable magnetic core, a saturating winding on said core, an electrical discharge tube provided with a control electrode supplying direct current to said saturating winding, a pair of first resistors respectively traversed by the anode current of said X-ray tubes, and having a common terminal, a channel comprising a rectifier in series with a second resistor shunted across each said first resistor, and means for impressing a voltage equal to the difference in the voltage drops through said second resistors on the control electrode of said discharge tube, to cause the latter to vary the direct current supplied to said saturating winding with attendant variation in the heating current supplied to the thermionic cathode of one of said X-ray tubes to maintain a constant anode current ratio of unity between said pair of X-ray tubes.

2. In combination with a pair of X-ray tubes each having a thermionically-emissive cathode heated by electric current, a circuit supplying heating current to one of said cathodes and embodying a. reactor having a saturable magnetic core, a saturating winding on said cor'e, an elec trical discharge tube provided with a control electrode supplying direct current to said saturating winding, a pair of first resistors respectively traversed by the anode current of said X-ray tubes, and having a common terminal, a channel comprising a rectifier in series with a second resistor, which is shunted by a capacitor, shunted across each said first resistor, and means for impressing a voltage equal to the difference in the voltage drops through said second resistors on the control electrode of said discharge tube, to cause the latter to vary the direct current supplied to said saturating winding with attendant variation in the heating current supplied to the thermionic cathode of one of said X-ray tubes to maintain a constant anode current ratio of unity between said pair of X-ray tubes.

3. In combination with a pair of electrical discharge tubes each having a cathode heated by electric current, a circuit for supplying current to one of said cathodes and embodying an impedance provided with a regulating winding, an electron tube provided with a control electrode controlling current flow through said winding, a pair of first resistors having a common terminal and respectively connected to carry the anode currents of said two electrical discharge tubes, each said first resistor being shunted by a channel comprising a rectifier in series with a second resistor, and means for impressing on said control electrode a voltage proportional to the difference in potential drops produced by current fiow through said rectifiers in said second resistors, to cause the latter to vary the direct current supplied to said saturating winding with attend-ant variation in the heating current supplied to the thermionic cathode of one of said X-ray tubes to maintain a constant anode current ratio of unity between said pair of X-ray tubes.

4. In combination with a pair of electrical discharge tubes each having a cathode heated by electric current, a circuit for supplying current to one of said cathodes and embodying an im pedance provided with a regulating winding, an electron tube provided with a control electrode controlling current flow through said winding, a pair of first resistors having a common terminal and respectively connected to carry the anode currents of said two electrical discharge tubes, each said first resistor being shunted by a channel comprising a rectifier in series with a second resistor which is shunted by a capacitor, and means for impressing on said control electrode a voltage proportional to the difference in potential drops produced by current flow through said rectifiers in said second resistors, to cause the latter to vary the direct current supplied to said saturating winding with attendant variation in the heating current supplied to the thermionic cathode of one of said X-ray tubes to maintain a constant anode current ratio of unity between said pair of X-r-ay tubes.

WALTER N. LUNDAHL.

ROBERT L. WRIGHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES X-ray Inspection and Phosphors and Photoelectric Tubes, by H. M. Smith, G. E. Review. vol. 48, No. 3, Mar. 1945, p. 17. 

